Many processes and devices have been used for electro photographic printers wherein a laser scan line is projected onto a photoconductive surface. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as the source of signals to be imaged on a pre-charged photoreceptor for purposes of xerographic printing. The ROS provides a laser beam which switches on and off as it moves, or scans, across the photoreceptor. Commonly, the surface of the photoreceptor is selectively imagewise discharged by the laser in locations to be printed, to form the desired image on the photoreceptor. On-and-off control of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling the laser source. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet, as in the well-known process of xerography.
Prior art laser scanning is based on techniques for achieving both start-of-scan detection and dynamic beam intensity regulation in a multiple laser beam raster output scanner using a single photodetector. The raster output scanner includes a source or sources of a plurality of laser beams, a rotating polygon having at least one reflecting facet for sweeping the laser beams to form a scan line path, and a photodetector for receiving illumination from the multiple laser beams and for converting those beams into beam-dependent electrical currents. The raster output scanner further includes a scan detection circuit for producing a start-of-scan signal from the beam dependent current, and a beam intensity circuit for producing an electrical output signal which depends upon the difference in beam intensity of at least two of the laser beams. Ideally, the raster output scanner also includes an optical fiber that collects a portion of the light flux in the sweeping laser beams which directs the light flux onto the photodetector.
A drawback in prior art laser scanning is typically with start of scan detectors (SOS) that have been used for line scan timing on Raster output Scanners (ROS). FIG. 1 illustrates a typical SOS 100 including a sensor pattern 104 through which a light beam 102 is passed. The TP1 106 and TP2 108 are configured with the sensor pattern and the comparator 110. The output 112 is interfaced with the traditional SOS. The graphical representation is shown with the input 114 and the graphs showing the curves of TP1 116 and TP2 118 and forming the output graph 120. Occasionally, SOS detectors are also used for power (μWatts, energy) detection in the output of ROS designs.
The production of a start-of-scan signal and the regulation of the intensities of multiple laser beams can be carried out independently, with separate photodetectors and separate preamplifiers for each. This substantially increases costs and manufacturing and assembly overhead while reducing overall system reliability. Therefore, a technique for achieving start-of-scan detection and dynamic beam intensity regulation of multiple element pre-patterned detectors using photodetector element would be beneficial. Furthermore, such a technique that can be used with a power detection function would be even more beneficial.